Quantitative respirator fit testing is required by the Nuclear Regulatory Commission for the health protection of workers in atomic installations, and by the Occupational Safety and Health Administration for workers exposed to lead, acrylonitrile, inorganic arsenic and coke oven emissions. In the conventional technique, di-2 (ethyl-hexyl) phthalate (DOP) has been used as the test aerosol, and the concentrations inside and outside the mask have been determined photometrically through measurements of the scattered light intensities from the DOP clouds. DOP, however, has recently been implicated as a potential human carcinogen. It is proposed that a continuous-flow condensation nuclei counter (CNC) be adapted for fit testing negative pressure respirators with replaceable cartridges. Also, a new testing protocol will be developed. The CNC enlarges submicrometer-sized, fine particles into the light scattering range, so virtually any non-hazardous cloud of fine particles can be used for fit testing. The use of a special aerosol electrometer will be studied as a potentially even simpler measurement method. While exploring new and simple fit testing techniques, protection factor will also be measured as a function of particle size since the performance of a respirator is expected to depend on the particle size distribution generated by the fit testing apparatus or the actual industrial environment. No method exists at present for fit testing disposable masks which are worn in the presence of silica, coal, or general nuisance dusts. It is proposed that an inert, nontoxic, supermicrometer-sized dust such as limestone be dispersed by means of a fluidized bed and that the dust concentration for several size ranges inside and outside the mask be determined through use of an aerodynamic particle size analyzer. The use of commercially available instruments is stressed in order to arrive at techniques that can be used immediately by the practitioner. The new fit test methods will be field-evaluated by measuring respirator fits in industrial environments.